As consumer secondary batteries used for mobile terminal devices, such as personal computers and mobile phones, video cameras, and the like, lithium ion batteries have been actively developed, which have high energy but can be downsized and formed in an ultra-thin shape.
As a packaging material for lithium ion batteries (hereinafter, also simply referred to as “packaging material”), laminated films with a multilayer structure have come to be used increasingly instead of a conventional metal can due to the advantages such as of having light weight and enabling choice of a desired battery shape. Packaging materials using such laminated film not only enables choice of a desired battery shape but also realizes light weight, high heat radiation protection and production at low cost. Therefore, such packaging materials are offered for applications to batteries for environment-friendly hybrid vehicles and electric vehicles which are increasingly developed in recent years.
Such a laminated film is typically configured by laminating a sealant layer (heat fusible film) to one surface of a metal foil layer, such as an aluminum foil, via an adhesive layer and laminating a substrate layer (plastic film) to the other surface via an adhesive layer (substrate layer/adhesive layer/metal foil layer/adhesive layer/sealant layer).
A lithium ion battery using a laminated film type packaging material is formed, for example, as follows. Firstly, the laminated film mentioned above is deep drawn using cold forming (deep drawing) to obtain a part. Then, an electrolytic solution or an electrolytic solution layer made of a polymer gel impregnated with an electrolytic solution is accommodated in the part together with a cathode material, an anode material, and a separator as a battery body. The part is formed into a product by heat sealing, with such members being accommodated therein.
As the electrolytic solution, an electrolytic solution obtained by dissolving a lithium salt in an aprotic solvent (propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc.) can be used.
The electrolytic solution easily permeates into a sealant layer. Therefore, in a lithium ion battery, the electrolytic solution that has permeated in the sealant layer reduces lamination strength between the metal foil layer and the sealant layer and can finally leak out. Lithium salts, such as LiPF6 and LiBF4, as electrolytes can generate hydrofluoric acid by hydrolysis reaction. Hydrofluoric acid causes corrosion in a metal surface and deteriorates lamination strength between layers of the laminated film. Therefore, packaging materials are required to have a corrosion prevention performance against electrolytic solutions and hydrofluoric acid.
As adhesives to adhere a sealant layer with a metal foil layer, polyurethane adhesives have been typically used. Such a polyurethane adhesive is obtained by formulating a polyfunctional isocyanate compound with a diol component.
However, the polyurethane adhesive is likely to be swollen with an organic solvent. In addition, a urethane bond forming the polyurethane adhesive is poor in durability to an electrolytic solution and hydrofluoric acid.
PTL 1, for example, discloses a packaging material that minimizes reduction in the lamination strength between a sealant layer and a metal foil layer caused by an electrolytic solution, and has sufficient electrolytic resistance. In this packaging material, a sealant layer and a metal foil layer are adhered via a layer made of an adhesive (adhesive layer) containing a polyolefin resin containing a carboxyl group and a polyfunctional isocyanate compound.